JPS5951077B2 - Non-volatile semiconductor memory - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a floating gate MOSFET type nonvolatile semiconductor memory.

The present invention uses a floating gate MOSFET type non-volatile semiconductor memory element by always applying a reverse bias to the electroded gate with respect to the substrate, thereby preventing the scattering of charges accumulated in the floating gate, and improving the memory element. The aim is to dramatically improve the non-volatility of

In recent years, integrated circuits have developed in the direction of higher density, smaller size, and energy savings.
In an IC circuit such as a C flip-flop circuit memory, I
It is becoming a memory element that can be manufactured at the same time as C.

MOSFETs perform amplification, switching, etc. by changing the electrical resistance between the drain and source by applying voltage to the gate electrode with respect to the substrate.In this case, applying voltage to the gate electrode Instead, a potential bowl is created in the gate insulating film and electrons are stored in it.
Using the electric effect of the accumulated electrons, MISFET
A general MISFET type memory element is used by turning it "ON". Here, if the electrons accumulated in the potential bowl provided in the gate insulating film do not leak and remain semi-permanently in the potential bowl, this MISFET type element will always remain in a conductive state. In other words, it is possible to manufacture the nonvolatile memory element simultaneously in a conventional IC substrate or by using substantially the same process as the IC manufacturing process.

Also, such a MISFET type memory element can be made to be approximately the same size as a MOS transistor, and since one element can be used as one bit, it is possible to make multiple bits extremely small. . There are two types of MISFET type non-volatile memory elements that have been considered up to now.One is that the gate insulating film is formed by overlapping two types of insulating materials in parallel with the silicon substrate. One is to generate a potential bowl in the interface of the insulating film of the layer and accumulate electrons in the potential bowl in the interface, and the other is called a floating gate in the gate I insulating film. , a material with good charge retention characteristics is provided isolated from the outside world, and electrons are accumulated in this floating gate, and the electric field effect of these electrons produces an effect equivalent to that of applying a voltage to the gate electrode of a MOS transistor. It is something that In this case, the floating gate is a polycrystalline silicon material, which is a semiconductor material.
Alternatively, silicon polycrystals with impurities diffused into P-type or N-type, and conductive metals are considered. However, in the former MISFET type non-volatile memory element, the area of the interface between the two layers of insulating films is larger than the area of the channel part.In other words, in addition to the effectively necessary two-layer interface, a potential bowl is created, and the area of the interface between the two layers is larger than the area of the channel part. If electrons move to areas other than the interface directly above, or where the two-layer interface is broken, electrons leak out of the element, making it impossible to achieve sufficient non-volatility of the memory element.

In the case of the latter type of non-volatile memory element, the electrons accumulated in the floating gate will not float as long as they are manufactured using planar technology, even if the floating gate is covered with an insulating film of the same material. The insulating film below the gate and the insulating film above the gate cannot be formed at the same time, and are made in two steps, mainly using SiO2, so the irregularity of the bonding interface can easily cause charge traps. , electrons can easily escape through that interface. Furthermore, in the case of a floating gate type, the amount of electrons escaping through the insulating film itself surrounding the floating gate poses a problem. Furthermore, the insulating film below the floating gate is made of relatively dense and high-quality SiO due to the manufacturing process. On the other hand, in the case of the upper insulating film, the SiO film is formed at low temperature using a gas phase reaction. A major problem is that the electrons accumulated in the floating gate will scatter over time through the upper insulating film.
The present invention provides a floating gate MISFET. This is a volatile memory element that eliminates these drawbacks and retains accumulated charges semi-permanently.

FIG. 1 is a cross-sectional view of a typical MOSFET, in which a drain 2 and a source 3 are diffused in a substrate 1, and a gate electrode 7 is provided in the channel portion via a gate insulating film 4.
Electrodes 8 and 6 are taken out from the drain and source, respectively.

When a voltage is applied to the gate electrode 7 with respect to the substrate, an inversion layer is generated in the channel portion, and the electrodes 6 and 8 become electrically conductive. In order to generate an inversion layer in the channel portion in this way, instead of applying voltage to the gate electrode, a floating gate is provided in the insulating film at the top of the channel, and charges are accumulated in this floating gate. A floating gate MISFET type nonvolatile memory element is a device that brings conduction between the drain and the source using an electric field effect, and FIG. 2 shows a cross-sectional structure of a typical device. 10 and 11 are source and drain diffusions, 12 is a gate insulating film, 13 is a floating gate, 14 is an SiO2 insulating film, and 15 and 16 are source and drain electrodes. Further, FIG. 3 is a cross-sectional view showing the structure of a floating gate MISFET type nonvolatile memory element embodying the present invention, in which 17 is a substrate, 18,
19 is a source and drain diffusion part, 20 is a gate insulating film, 21 is a floating gate, 22 is an SiO insulating film formed by a gas phase reaction, 23 is a gate with an electrode AL24 and 25 are a source and drain electrode, respectively. be. In this memory element, the gate 23 with an electrode is always biased in the opposite direction to the substrate 17, thereby preventing the charges accumulated in the floating 21 from escaping through the insulating film 22 above the floating gate. , it is possible to semipermanently retain the charge on the floating gate. In other words, in the case of a P channel type, an N type substrate is used as the substrate 17, and P type impurities are diffused into the source and drain 19.
Electrons are injected into the floating gate 21 by this avalanche breakdown of the PN junction. When using this device, if the gate 23 with electrode is always biased to a negative voltage with respect to the substrate 17, the electric field will prevent the electrons in the floating gate from escaping towards the insulating film 22. The electron retention properties are extremely good without any
It is believed that the nonvolatility of this memory element will be dramatically increased. Floating gate MI of the structure in the present invention
SFET-type non-volatile memory elements can be produced using almost the same manufacturing process as conventional floating-gate MISFET-type non-volatile memory elements, and the non-volatile characteristics can be maintained even with almost no changes to the manufacturing equipment. It is possible to manufacture a good memory element.

This is similarly effective in the case of the N-channel type. In this case, positive charges are stored in the floating gate and used, so in order to retain the charges, the electroded gate 23 must be It is assumed that a positive voltage is applied to the substrate 17 during use. Furthermore, the larger the electroded gate 23 is, the greater the effect of preventing charge scattering, and if it is made as large as possible, the non-volatile characteristics will be improved. As described above, in the present invention, since a reverse bias voltage is applied to the gate electrode with respect to the semiconductor substrate during storage, the charge accumulated in the floating gate is prevented from scattering, and the non-volatile state of the memory element is improved. can be dramatically improved.

[Brief explanation of the drawing]

FIG. 1 shows a cross-sectional view of one example of a typical MOSFET, and FIG. 2 shows a cross-sectional view of a typical structure of a conventional floating gate MISFET type nonvolatile memory element. FIG. 3 is a sectional view showing an example of the structure of a nonvolatile memory element embodying the present invention. 1, 9, 17... substrate, 2, 10, 18...
... Source diffusion part, 3, 11, 19... Drain diffusion part, 4, 12, 20... Gate insulating film, 5... Oxide insulating film, 14, 22...
・・CVD-SiO2 insulation coating, 13, 21...
・Floating gate, 8, 15, 24...
Source electrode, 6, 16, 25...Drain electrode, 7...Gate electrode, 23...Gate with electrode.

Claims (1)

[Claims] 1. A source/drain provided in a semiconductor substrate, a first insulating film provided on a channel between the source/drain, a floating gate provided on the first insulating film, and a device formed by covering the floating gate. a second insulating film;
A nonvolatile semiconductor memory comprising a gate electrode provided on the second insulating film, wherein a reverse bias voltage is applied to the gate electrode with respect to the semiconductor substrate during memory retention. sexual semiconductor memory.